Re: POLLPRI/poll() behavior change since 2.6.31

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 06 janvier 2011 à 13:50 -0200, Leonardo Chiquitto a écrit :
> Hello,
> 
> Since 2.6.31, poll() no longer returns when waiting exclusively for a
> POLLPRI event. If we wait for POLLPRI | POLLIN, though, it will
> correctly return POLLPRI as a received event.
> 
> The reproducer (code below) will print the following when running on
> 2.6.30:
> 
> $ ./pollpri-oob 
> main: starting
> main: setup_pipe ok (sfd[0] = 5, sfd[1] = 4)
> parent: child <pid 3790> started
> child: polling...
> parent: sending the message
> parent: waiting for child to exit
> child: poll(): some data <1,2> has arrived!
> child: done
> parent: done
> 
> ... and will block when running on 2.6.37-rc7:
> 
> $ ./pollpri-oob 
> main: starting
> main: setup_pipe ok (sfd[0] = 5, sfd[1] = 4)
> parent: child <pid 14148> started
> child: polling...
> parent: sending the message
> parent: waiting for child to exit
> [hangs here]
> 
> I've bisected this behavior change to the following commit:
> 
> commit 4938d7e0233a455f04507bac81d0886c71529537
> Author: Eric Dumazet <dada1@cosmosbay.com>
> Date:   Tue Jun 16 15:33:36 2009 -0700
> 
>   poll: avoid extra wakeups in select/poll
> 
>   After introduction of keyed wakeups Davide Libenzi did on epoll, we are
>   able to avoid spurious wakeups in poll()/select() code too.
> 
>   For example, typical use of poll()/select() is to wait for incoming
>   network frames on many sockets.  But TX completion for UDP/TCP frames call
>   sock_wfree() which in turn schedules thread.
> 
>   When scheduled, thread does a full scan of all polled fds and can sleep
>   again, because nothing is really available.  If number of fds is large,
>   this cause significant load.
> 
>   This patch makes select()/poll() aware of keyed wakeups and useless
>   wakeups are avoided.  This reduces number of context switches by about 50%
>   on some setups, and work performed by sofirq handlers.
> 
> 
> I don't know if the new behavior is correct, but we've got one report of
> an application that broke due to the change.

Hi Leonardo

Hmm, this is because 	sock_def_readable() uses :

wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLRDNORM |
POLLRDBAND);

So POLLPRI bit is not signaled. 

I would just add POLLPRI flag in sock_def_readable()

(Alternatively, define a tcp_def_readable() function to pass POLLPRI
only if TCP_URG is set, but is it worth the pain for a seldom used
feature ?)

David, do you have an opinion on this ?

Thanks

diff --git a/net/core/sock.c b/net/core/sock.c
index e5af8d5..7fd3541 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1907,7 +1907,7 @@ static void sock_def_readable(struct sock *sk, int len)
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
 	if (wq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, POLLIN |
+		wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLPRI |
 						POLLRDNORM | POLLRDBAND);
 	sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
 	rcu_read_unlock();